TOKYO — The burned insides of a battery in the Boeing 787 Dreamliner at the center of a worldwide grounding of the aircraft indicate it operated at a voltage above its design limit, a Japanese investigator said Friday as U.S. officials joined Japan’s probe into the incident.

The All Nippon Airways plane made an emergency landing Wednesday morning in western Japan after its pilots smelled something burning and received a cockpit warning of battery problems.

The 50 Dreamliners in use around the world have since been grounded, and Boeing confirmed Friday it will not deliver any 787s until the company and federal regulators agree the battery problems have been corrected.

Photos provided by the Japan Transport Safety Board of the lithium-ion battery that was located beneath the ANA 787’s cockpit show a blackened mass of wires and other components within a distorted blue casing.

Japan transport-ministry investigator Hideyo Kosugi said the state of the battery indicated “voltage exceeding the design limit was applied” to it.

He said the similarity of the burned insides of the battery from the ANA flight to the battery in a Japan Airlines 787 that caught fire Jan. 7 while the jet was parked at Boston’s Logan International Airport suggested a common cause.

“If we compare data from the latest case here and that in the U.S., we can pretty much figure out what happened,” Kosugi said.

The 787 relies more than any other modern airliner on electricity to power nearly everything the plane does. It’s also the first Boeing plane to use rechargeable lithium-ion batteries for its main electrical system. Such batteries are prone to overheating but have additional safeguards installed that are meant to control the problem and prevent fires.

GS Yuasa Corp., the maker of the lithium-ion batteries used in the 787s, said Thursday it was helping with the investigation but that the cause of the problem was unclear. It said the problem could be the battery, the power source or the electronics system.

U.S. safety officials and Boeing inspectors joined the Japan Transport Safety Board investigation Friday. The American investigators — one each from the Federal Aviation Administration and the National Transportation Safety Board, and two from Boeing — inspected the ANA jet on the tarmac at Takamatsu airport in western Japan.

An initial inspection by Japanese officials of the 787 found that a flammable-battery fluid known as electrolyte had leaked from the plane’s main lithium-ion battery beneath the cockpit. It also found burn marks around the battery.

Some aviation-safety and battery experts said Friday the damage could have been caused by overcharging.

Lithium-ion batteries are particularly vulnerable to igniting if they receive too much voltage too fast, experts said. Other types of batteries may overheat in those circumstances, but they are far less susceptible to starting a fire, they said.

“Other batteries don’t go this wrong when you treat them this badly,” said Jay Whitacre, an associate professor of materials science and engineering at Carnegie Mellon University. “The overall story here is these batteries are full of flammable electrolyte and they will burn if they are mistreated and something goes wrong.”

There was one lithium-ion-battery fire during testing of the batteries while Boeing was working with the FAA on certification of the 787, said Marc Birtel, a spokesman for the aircraft maker.

However, that fire was due to problems with the test rather than the batteries themselves, he said.

“There are multiple backups to ensure the system is safe,” Birtel said. “These include protections against overcharging and over-discharging.”

But John Goglia, an aviation-safety expert and former National Transportation Safety Board member, said, “It certainly sounds like, based on what has been released so far, that we have an issue of the battery charger or some other source providing too much energy to the battery.”

He said too-rapid charging might cause the electrolyte fluid in the batteries to overheat, leak and catch fire.

If the incidents are due to overcharging batteries, that might be good news for Boeing, Goglia said. A flaw in the aircraft’s electronics that permits overcharging would likely be much easier to fix than having to replace the aircraft’s lithium batteries with another kind of battery, he said.

Another possibility is a manufacturing defect in the batteries themselves, Whitacre said. More than other types of batteries, lithium-ion batteries rely on very thin sheets of material internally to separate the negative and positive poles. The slightest flaw can cause a short circuit, overheating the flammable electrolytes.

“It’s a delicate ecosystem that you are creating inside it, and you have to manufacture it with perfect integrity,” Whitacre said. “Then you have to keep it in the right voltage range and be very safe with its environmental conditions.”

Jim McNerney, Boeing’s chairman, president and CEO, sent the company’s employees a letter Friday expressing confidence in the 787 and vowing to return the plane to service. “I remain tremendously proud of employees across the company for the decade of effort that has gone into designing, developing, building and delivering the most innovative commercial airplane ever imagined,” he said.